1. Field of the Invention
The present invention relates to an image forming apparatus utilizing electrophotographic process and, more specifically, to an image forming apparatus in which image quality can be adjusted.
2. Description of the Related Art
The electrophotographic process includes a charging process for uniformly charging the surface of a photoreceptor, an exposure process for exposing the surface of the photoreceptor in response to image information for partially removing charges and forming a latent image, a developing process of sticking toner which is contained in a developer to the latent image for forming a toner image, transfer process for transferring the toner image onto a recording paper (hereinafter referred to as "paper"), and a fixing process for fixing the toner image transferred onto the paper, and it is widely used as a method for forming a hard copy image. Image forming apparatuses utilizing the electrophotographic process includes copying machines, facsimiles, and optical printers using a laser or a LED array as a light source.
In such an image forming apparatus, operation levels of various image forming devices (mechanical parts) such as a corona charger, a light source for exposure, a developer are automatically set (hereinafter this setting is referred to as "image adjustment"), so as to provide hard copy images of desired quality. Namely, property values of the electrophotographic process, for example surface potential of the photoreceptor, density of the toner image are susceptible to various environmental influence such as temperature and moisture. The property values are also changed by degradation with time of the respective image forming devices. Therefore, generally, output values of the respective image forming devices are determined at the time of power on, so as to provide the property values corresponding to predetermined appropriate (standard) image quality, in view of the environmental conditions and states of various image forming devices at that time.
A method for forming an image in a conventional image forming apparatus is disclosed in, for example, Japanese Patent Laying-Open No. 55-17174. According to this article, in image adjustment, operation levels of the various image forming devices are changed stepwise. Property values at respective stages are measured, and based on the relation between the operation levels and the property values, an operation level corresponding to a prescribed property value is found by calculation.
Such an image forming apparatus takes shorter time for image adjustment, compared with an image forming apparatus in which image adjustment is done by a so called feed back control in which the operation level is gradually changed to the optimal value while the property values are measured, and therefore image formation becomes possible immediately after the power on.
The above described image adjustment is carried out not only at the time of power on but also at other occasions. For example, setting of the operation levels to provide desired image quality is done when a dark image or a bright image is designated by an operator through a density selecting key, when an image forming mode suitable for a half tone image such as a photograph or suitable for a color image is designated, or some initialization is done after a trouble such as jamming is recovered.
Sometimes, the operation levels are set a plural times in one image adjusting operation. An operation level of one image forming device is temporarily set. Then an operation level of another image forming device is optimized, and the temporarily set operation level of the image device is set again. Namely, generally, setting of the operation levels is carried out a plural times after power on.
However, in a conventional image forming apparatus, measurement of property values is done every time the operation level is set, so that it takes long to set the operation levels at each time.